Principia College Astronomical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Principia College Astronomical Observatory clear sky chart from 2005-11-06 to 2024-01-15.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	12% 7% 3 6% 5% 3 5% 5% 5% 8% 42%	74765
	Night	13% 7% 3 6% 5% 3 4 4 5% 7% 43%	75028
Transparency	Day	27% 38% 8% 16% 10% 2	74473
	Night	28% 37% 7% 15% 11% 2	74842
Seeing	Day	14% 9% 26% 27% 18% 6%	14485
	Night	16% 9% 29% 31% 14% 1	24440

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	11% 11% 6% 9% 5% 3 5% 4 4 7% 34%	1517
	11th to 20th	Day	16% 10% 6% 8% 8% 4 4 4 4 6% 31%	1566
	21st to end	Day	13% 11% 5% 9% 6% 3 3 3 4 6% 37%	1782
February	1st to 10th	Day	16% 9% 6% 8% 5% 3 4 5% 5% 6% 33%	1620
	11th to 20th	Day	14% 9% 4 5% 7% 4 4 5% 5% 7% 35%	1729
	21st to end	Day	16% 11% 4 7% 5% 3 4 4 5% 5% 38%	1480
March	1st to 10th	Day	15% 8% 4 5% 6% 3 5% 4 4 6% 41%	1814
	11th to 20th	Day	18% 9% 4 5% 6% 3 4 5% 5% 6% 35%	1980
	21st to end	Day	20% 10% 4 7% 6% 3 5% 5% 5% 6% 28%	2296
April	1st to 10th	Day	12% 6% 2 6% 6% 4 5% 4 5% 8% 42%	2160
	11th to 20th	Day	16% 8% 3 6% 5% 2 4 4 5% 5% 40%	2134
	21st to end	Day	17% 7% 2 6% 6% 3 4 4 4 6% 38%	2160
May	1st to 10th	Day	15% 6% 3 6% 5% 3 5% 5% 6% 8% 37%	2227
	11th to 20th	Day	16% 5% 3 5% 6% 4 5% 5% 6% 8% 39%	2456
	21st to end	Day	10% 6% 2 6% 6% 4 7% 6% 6% 9% 38%	2725
June	1st to 10th	Day	8% 4 3 4 4 3 5% 5% 6% 9% 48%	2520
	11th to 20th	Day	8% 4 2 4 5% 3 5% 5% 7% 9% 48%	2520
	21st to end	Day	5% 3 2 4 5% 3 6% 6% 8% 12% 46%	2520
July	1st to 10th	Day	8% 4 2 5% 5% 4 6% 5% 7% 10% 43%	2520
	11th to 20th	Day	4 3 1 4 4 4 5% 6% 6% 9% 53%	2520
	21st to end	Day	8% 4 2 4 4 3 5% 6% 6% 9% 49%	2635
August	1st to 10th	Day	8% 4 2 6% 6% 3 6% 6% 7% 10% 42%	2324
	11th to 20th	Day	9% 5% 2 6% 6% 4 7% 6% 6% 10% 41%	2125
	21st to end	Day	7% 4 2 4 5% 3 5% 5% 6% 8% 50%	2376
September	1st to 10th	Day	13% 5% 2 4 3 2 4 3 5% 8% 50%	2160
	11th to 20th	Day	8% 4 2 5% 4 3 4 4 5% 8% 52%	2134
	21st to end	Day	6% 5% 2 4 5% 3 3 4 5% 7% 55%	1980
October	1st to 10th	Day	11% 4 1 4 5% 2 4 4 4 6% 54%	1800
	11th to 20th	Day	16% 6% 3 4 4 2 3 3 4 6% 50%	1800
	21st to end	Day	17% 8% 3 6% 3 2 3 3 4 6% 45%	1949
November	1st to 10th	Day	11% 4 2 4 4 2 3 3 4 7% 54%	1672
	11th to 20th	Day	15% 7% 3 5% 4 2 4 3 4 6% 47%	1710
	21st to end	Day	15% 11% 4 6% 5% 3 4 3 4 6% 40%	1633
December	1st to 10th	Day	13% 11% 4 7% 4 3 5% 4 4 6% 39%	1520
	11th to 20th	Day	14% 9% 4 5% 6% 3 5% 4 5% 6% 38%	1918
	21st to end	Day	20% 14% 5% 8% 6% 3 4 5% 4 5% 24%	2783
January	1st to 10th	Night	12% 9% 5% 8% 5% 4 5% 4 4 6% 38%	2648
	11th to 20th	Night	16% 9% 4 9% 6% 3 4 4 3 6% 35%	2478
	21st to end	Night	14% 11% 4 8% 5% 4 5% 4 5% 7% 34%	2574
February	1st to 10th	Night	15% 12% 6% 7% 6% 3 4 4 5% 5% 33%	2340
	11th to 20th	Night	16% 7% 4 6% 6% 3 4 3 6% 5% 41%	2116
	21st to end	Night	17% 10% 3 7% 5% 2 3 3 4 5% 40%	1776
March	1st to 10th	Night	16% 9% 3 5% 5% 3 5% 3 4 5% 42%	2160
	11th to 20th	Night	19% 7% 4 6% 5% 3 4 4 5% 7% 36%	1980
	21st to end	Night	21% 10% 3 5% 5% 3 5% 5% 5% 7% 31%	2059
April	1st to 10th	Night	12% 6% 3 5% 5% 2 4 5% 5% 7% 46%	1800
	11th to 20th	Night	13% 7% 3 4 5% 3 4 5% 4 7% 45%	1775
	21st to end	Night	18% 6% 4 7% 5% 2 4 4 4 6% 41%	1800
May	1st to 10th	Night	16% 7% 3 3 5% 2 4 5% 5% 7% 43%	1732
	11th to 20th	Night	13% 7% 4 6% 4 3 4 4 5% 7% 45%	1435
	21st to end	Night	8% 6% 3 5% 6% 4 5% 6% 7% 9% 42%	1551
June	1st to 10th	Night	8% 4 3 5% 4 2 5% 4 7% 9% 50%	1440
	11th to 20th	Night	6% 4 1 4 5% 2 4 5% 6% 9% 53%	1440
	21st to end	Night	5% 4 2 5% 4 3 4 5% 8% 10% 51%	1440
July	1st to 10th	Night	9% 4 2 4 4 2 5% 5% 6% 10% 50%	1440
	11th to 20th	Night	4 3 2 4 5% 3 5% 4 6% 10% 54%	1440
	21st to end	Night	6% 4 3 5% 5% 3 5% 4 7% 8% 51%	1679
August	1st to 10th	Night	7% 4 1 3 4 3 5% 6% 6% 8% 53%	1615
	11th to 20th	Night	6% 5% 2 5% 4 3 4 5% 5% 8% 52%	1776
	21st to end	Night	5% 3 1 3 5% 3 4 5% 5% 9% 57%	1980
September	1st to 10th	Night	11% 4 2 5% 4 2 4 4 4 8% 52%	1800
	11th to 20th	Night	8% 4 2 5% 5% 2 4 4 6% 8% 51%	1808
	21st to end	Night	8% 4 2 3 4 3 4 5% 5% 6% 56%	1980
October	1st to 10th	Night	9% 4 2 4 5% 2 3 4 4 7% 56%	2155
	11th to 20th	Night	14% 6% 2 3 4 2 3 4 5% 6% 51%	2160
	21st to end	Night	18% 7% 2 4 4 2 4 4 4 5% 45%	2406
November	1st to 10th	Night	9% 5% 2 4 4 2 4 4 4 6% 56%	2406
	11th to 20th	Night	13% 6% 3 5% 4 2 4 3 5% 7% 49%	2470
	21st to end	Night	18% 7% 3 6% 4 3 3 3 4 5% 44%	2546
December	1st to 10th	Night	15% 10% 4 7% 6% 3 5% 5% 6% 6% 34%	2660
	11th to 20th	Night	17% 8% 4 6% 4 2 4 3 4 6% 42%	3279
	21st to end	Night	20% 19% 5% 8% 6% 3 5% 4 3 5% 21%	4884

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	32% 40% 7% 14% 7%	1518
	11th to 20th	Day	32% 42% 7% 13% 4	1566
	21st to end	Day	36% 36% 8% 15% 5% 1	1782
February	1st to 10th	Day	34% 37% 8% 14% 6%	1620
	11th to 20th	Day	30% 37% 8% 14% 10% 1	1739
	21st to end	Day	32% 36% 8% 16% 7% 2	1480
March	1st to 10th	Day	29% 37% 6% 18% 10% 1	1814
	11th to 20th	Day	34% 37% 8% 12% 7% 1	1980
	21st to end	Day	38% 37% 8% 11% 5% 1	2296
April	1st to 10th	Day	24% 37% 8% 19% 13%	2160
	11th to 20th	Day	31% 36% 5% 15% 11% 3	2134
	21st to end	Day	30% 38% 9% 15% 7%	2160
May	1st to 10th	Day	31% 39% 8% 15% 6% 1	2227
	11th to 20th	Day	29% 39% 7% 13% 11% 1	2460
	21st to end	Day	29% 40% 9% 12% 9% 2	2725
June	1st to 10th	Day	21% 40% 10% 18% 11% 1	2520
	11th to 20th	Day	18% 39% 10% 14% 17% 2	2520
	21st to end	Day	19% 38% 10% 15% 13% 5%	2520
July	1st to 10th	Day	21% 41% 9% 17% 9% 3	2451
	11th to 20th	Day	18% 40% 10% 17% 13% 2	2445
	21st to end	Day	22% 40% 9% 18% 9% 2	2489
August	1st to 10th	Day	21% 40% 8% 18% 12%	2310
	11th to 20th	Day	22% 38% 11% 17% 11% 1	2125
	21st to end	Day	18% 37% 8% 18% 14% 5%	2376
September	1st to 10th	Day	24% 37% 6% 15% 14% 4	2160
	11th to 20th	Day	21% 37% 5% 19% 16% 1	2134
	21st to end	Day	19% 37% 6% 18% 17% 2	1980
October	1st to 10th	Day	17% 35% 6% 19% 21% 2	1800
	11th to 20th	Day	24% 36% 8% 16% 13% 3	1800
	21st to end	Day	27% 37% 6% 18% 10% 2	1949
November	1st to 10th	Day	18% 40% 10% 17% 13% 3	1672
	11th to 20th	Day	24% 39% 7% 16% 11% 2	1710
	21st to end	Day	27% 40% 8% 15% 9%	1633
December	1st to 10th	Day	29% 40% 9% 16% 5%	1520
	11th to 20th	Day	33% 33% 7% 17% 8% 1	1915
	21st to end	Day	49% 27% 9% 9% 6% 1	2783
January	1st to 10th	Night	31% 40% 7% 15% 6%	2648
	11th to 20th	Night	33% 41% 6% 14% 6%	2478
	21st to end	Night	35% 37% 8% 12% 7% 1	2574
February	1st to 10th	Night	37% 36% 6% 13% 7%	2340
	11th to 20th	Night	30% 37% 7% 14% 10% 2	2117
	21st to end	Night	32% 35% 8% 17% 6% 2	1776
March	1st to 10th	Night	29% 37% 6% 16% 12% 1	2160
	11th to 20th	Night	34% 37% 8% 14% 6% 2	1980
	21st to end	Night	38% 37% 9% 10% 5%	2059
April	1st to 10th	Night	22% 37% 7% 19% 13% 2	1800
	11th to 20th	Night	27% 36% 6% 15% 13% 3	1775
	21st to end	Night	30% 37% 6% 15% 11% 1	1800
May	1st to 10th	Night	29% 38% 9% 13% 10% 1	1732
	11th to 20th	Night	27% 38% 8% 14% 12% 2	1436
	21st to end	Night	24% 40% 10% 12% 11% 3	1552
June	1st to 10th	Night	19% 39% 10% 19% 12% 2	1440
	11th to 20th	Night	15% 38% 10% 18% 16% 3	1440
	21st to end	Night	18% 39% 8% 15% 14% 6%	1440
July	1st to 10th	Night	19% 40% 9% 19% 11% 3	1400
	11th to 20th	Night	16% 39% 10% 20% 12% 3	1392
	21st to end	Night	22% 40% 8% 18% 11% 2	1586
August	1st to 10th	Night	16% 40% 9% 20% 13% 2	1605
	11th to 20th	Night	19% 38% 9% 18% 15% 1	1776
	21st to end	Night	14% 37% 8% 19% 17% 5%	1980
September	1st to 10th	Night	23% 37% 6% 14% 14% 7%	1800
	11th to 20th	Night	22% 37% 5% 16% 16% 3	1808
	21st to end	Night	19% 36% 6% 15% 20% 4	1980
October	1st to 10th	Night	16% 37% 6% 17% 20% 4	2155
	11th to 20th	Night	23% 37% 5% 18% 12% 5%	2160
	21st to end	Night	28% 37% 5% 14% 13% 3	2406
November	1st to 10th	Night	17% 41% 8% 17% 14% 3	2406
	11th to 20th	Night	24% 40% 6% 17% 12% 2	2470
	21st to end	Night	26% 39% 8% 15% 11% 1	2546
December	1st to 10th	Night	32% 40% 7% 17% 4	2660
	11th to 20th	Night	32% 33% 8% 16% 11%	3281
	21st to end	Night	53% 26% 7% 9% 4	4884

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	21% 17% 36% 19% 6%	270
	11th to 20th	Day	27% 16% 37% 17% 3	252
	21st to end	Day	17% 20% 43% 17% 3	264
February	1st to 10th	Day	24% 17% 41% 17% 2	240
	11th to 20th	Day	22% 23% 32% 19% 4 1	240
	21st to end	Day	26% 16% 38% 18% 2	198
March	1st to 10th	Day	18% 17% 44% 18% 3	240
	11th to 20th	Day	29% 15% 33% 18% 4	240
	21st to end	Day	30% 14% 35% 16% 6%	382
April	1st to 10th	Day	15% 13% 41% 20% 11% 1	420
	11th to 20th	Day	20% 16% 28% 23% 12% 1	412
	21st to end	Day	21% 15% 32% 23% 8% 2	420
May	1st to 10th	Day	18% 12% 30% 29% 11% 1	423
	11th to 20th	Day	18% 8% 28% 26% 17% 4	602
	21st to end	Day	12% 6% 28% 31% 16% 7%	679
June	1st to 10th	Day	7% 6% 19% 33% 26% 9%	630
	11th to 20th	Day	9% 2 22% 35% 25% 7%	630
	21st to end	Day	5% 5% 27% 34% 22% 7%	630
July	1st to 10th	Day	10% 2 13% 27% 32% 17%	621
	11th to 20th	Day	6% 3 15% 36% 30% 10%	618
	21st to end	Day	9% 3 14% 32% 31% 11%	684
August	1st to 10th	Day	8% 2 13% 36% 31% 10%	622
	11th to 20th	Day	13% 3 13% 26% 33% 12%	439
	21st to end	Day	8% 1 13% 28% 36% 14%	495
September	1st to 10th	Day	12% 4 15% 30% 27% 11%	450
	11th to 20th	Day	8% 3 13% 31% 31% 14%	450
	21st to end	Day	8% 7% 22% 30% 24% 9%	450
October	1st to 10th	Day	9% 10% 24% 37% 16% 4	270
	11th to 20th	Day	15% 18% 32% 23% 11% 1	270
	21st to end	Day	27% 19% 27% 21% 7%	297
November	1st to 10th	Day	12% 9% 36% 24% 14% 4	270
	11th to 20th	Day	19% 17% 31% 19% 11% 2	270
	21st to end	Day	22% 14% 30% 26% 7% 1	270
December	1st to 10th	Day	11% 16% 42% 19% 7% 4	270
	11th to 20th	Day	18% 17% 34% 26% 4	270
	21st to end	Day	24% 19% 39% 15% 4	297
January	1st to 10th	Night	17% 14% 38% 25% 5%	950
	11th to 20th	Night	21% 14% 36% 23% 5%	921
	21st to end	Night	19% 13% 39% 27% 2	990
February	1st to 10th	Night	21% 13% 39% 23% 3	900
	11th to 20th	Night	20% 14% 39% 25% 2	721
	21st to end	Night	22% 15% 36% 25% 2	592
March	1st to 10th	Night	22% 13% 37% 25% 4	720
	11th to 20th	Night	23% 10% 33% 28% 6%	540
	21st to end	Night	25% 10% 33% 27% 5%	594
April	1st to 10th	Night	16% 11% 40% 26% 7%	540
	11th to 20th	Night	16% 9% 36% 29% 10% 1	533
	21st to end	Night	21% 8% 35% 29% 7% 1	540
May	1st to 10th	Night	16% 9% 29% 36% 9% 1	540
	11th to 20th	Night	16% 6% 25% 35% 16% 1	531
	21st to end	Night	8% 5% 24% 41% 20% 3	584
June	1st to 10th	Night	9% 2 20% 38% 31% 1	540
	11th to 20th	Night	8% 2 19% 47% 22% 2	540
	21st to end	Night	5% 2 22% 44% 26% 2	540
July	1st to 10th	Night	11% 2 8% 35% 36% 8%	525
	11th to 20th	Night	4 2 14% 43% 34% 3	522
	21st to end	Night	7% 1 12% 41% 36% 3	591
August	1st to 10th	Night	8% 1 9% 44% 34% 3	537
	11th to 20th	Night	7% 1 12% 40% 35% 4	534
	21st to end	Night	5% 1 12% 37% 40% 5%	594
September	1st to 10th	Night	14% 1 13% 37% 30% 6%	540
	11th to 20th	Night	8% 3 15% 34% 35% 5%	548
	21st to end	Night	9% 5% 22% 33% 27% 4	720
October	1st to 10th	Night	10% 4 26% 32% 23% 4	720
	11th to 20th	Night	17% 10% 31% 30% 12% 1	720
	21st to end	Night	23% 11% 30% 29% 6%	792
November	1st to 10th	Night	12% 8% 37% 32% 10% 1	740
	11th to 20th	Night	15% 14% 34% 29% 7%	760
	21st to end	Night	22% 12% 34% 27% 6%	836
December	1st to 10th	Night	20% 12% 35% 27% 5% 1	950
	11th to 20th	Night	23% 8% 31% 32% 5%	950
	21st to end	Night	27% 15% 32% 23% 3	1045

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